1. Field of the Invention
The present invention relates to a magneto-optical disk recording apparatus and more particularly to a magneto-optical disk recording apparatus capable of overwriting along a plurality of recording tracks using a plurality of laser beams emitted from a plurality of laser beam sources.
2. Description of the Prior Art
In a prior art magneto-optical disk recording apparatus, information desired to be recorded is magnetically recorded making use of the property of a magneto-optical disk comprising a perpendicular magnetic film with a perpendicular magnetic anisotropy exhibiting a great change in its magnetization characteristics when it is heated beyond the Curie temperature T.sub.C.
More particularly, the perpendicular magnetic film on a magneto-optical disk has such a magnetization characteristic that its coercive force H.sub.C sharply decreases at the Curie temperature T.sub.C, and hence, by applying an external magnetic field to the perpendicular magnetic film in its state first heated above the Curie temperature T.sub.C and then cooling the magneto-optical disk below the Curie temperature T.sub.C, the direction of magnetization in the perpendicular magnetic film can be aligned along the direction of the external magnetic field.
Based upon such principle of magnetization, in a magneto-optical disk recording apparatus of a magnetic field modulation type capable of the so-called overwrite, i.e., writing in new information while destroying information in a memory location, a light beam is continuously thrown on the magneto-optical disk to scan it with a spot of the beam to thereby heat a predetermined region of it above Curie temperature T.sub.C, and at the same time, a modulating magnetic field whose polarity reverses according to information to be recorded and which is timed by a reference clock is applied to the region using a magnetic head.
Through such operation, the region irradiated by the light spot and heated above the Curie temperature T.sub.C is naturally cooled when the light spot moves past it, and then, the region, when it is cooled below the Curie temperature T.sub.C, remains magnetized in the direction of the modulating magnetic field that was applied thereto.
Thus, micro regions aligned along the direction of the modulating magnetic field are sequentially formed on the magneto-optical disk along the locus scanned by the light spot, even if there is already formed a magnetic pattern, in synchronism with the cycle of the reference clock, and thereby, the overwrite of the information desired to be recorded on the magneto-optical disk is achieved.
In such a magneto-optical disk recording apparatus of a magnetic field modulation type, when it is intended to increase recording and reproducing speed of the record information (the so-called transfer rate), it is considered desirable to modulate the magnetic head at a higher frequency.
When it is tried to put the above idea into practice, however, it becomes difficult to have a sufficient length of time to cool the magneto-optical disk and hence a bit error of record information occurs, thus making it difficult to put that idea into practice.
Therefore, in a magneto-optical disk recording apparatus of a light modulation recording type, as it is called, in which a light beam is ON-OFF modulated according to record information while the magneto-optical disk is subjected to a recording magnetic field with a fixed direction of magnetization, there is proposed an apparatus enabled to provide an increased transfer rate of the record information employing the so-called multi-beam system.
More particularly, in such a magneto-optical disk recording apparatus of a multi-beam system, the laser beam source is formed of a plurality of semiconductor laser elements in a hybrid or a monolithic arrangement, whereby a plurality of light beams independently modulated according to the respective pieces of information to be recorded are emitted to irradiate a plurality of adjoining recording tracks on the magneto-optical disk through a common optical system.
Thus, it is designed to offer an increased transfer rate of the record information by the capability of recording, and reproducing, the information along the plurality of recording tracks at, practically, the same time.
However, since the recording magnetic field of a fixed direction of magnetization is adapted to be applied to the magneto-optical disk in the above case, it is required to perform an erasing process along the recording track prior to the recording of information. Accordingly, an additional processing time is required. Therefore, it has not been advantageous to put this design into practice.